400 nm-induced Br₂ dissociation dynamics probed by the 90.6 eV FEL pulse

Figure 5. 400 nm-induced Br₂ dissociation dynamics probed by the 90.6 eV FEL pulse. (a)–(c) Experimentally recorded Br^{2 +} 2D momentum distributions for selected pump–probe delays; (a) when the FEL pulse precedes the 400 nm pulse by 0.9 ps, (b) when the 400 nm pulse precedes the FEL pulse by 0.3 ps and (c) when the 400 nm pulse precedes the FEL pulse by 1.8 ps. In all cases the 400 nm laser pulse followed 1 ps after an 800 nm laser pulse that dynamically aligned the Br₂ sample. (d), (e) The delay-integrated and time-dependent Br^{2 +} kinetic energy distributions. (f) The signal strength and the degree of alignment for the dissociative channel at 0.65 eV. The black dotted line is a Gauss error function fitted to the ionization yield with a rise time of 340 fs.

Abstract

The dissociation dynamics induced by a 100 fs, 400 nm laser pulse in a rotationally cold Br₂ sample was characterized by Coulomb explosion imaging (CEI) using a time-delayed extreme ultra-violet (XUV) FEL pulse, obtained from the Free electron LASer in Hamburg (FLASH). The momentum distribution of atomic fragments resulting from the 400 nm-induced dissociation was measured with a velocity map imaging spectrometer and used to monitor the internuclear distance as the molecule dissociated. By employing the simultaneously recorded in-house timing electro-optical sampling data, the time resolution of the final results could be improved to 300 fs, compared to the inherent 500 fs time-jitter of the FEL pulse. Before dissociation, the Br₂ molecules were transiently 'fixed in space' using laser-induced alignment. In addition, similar alignment techniques were used on CO₂ molecules to allow the measurement of the photoelectron angular distribution (PAD) directly in the molecular frame (MF). Our results on MFPADs in aligned CO₂ molecules, together with our investigation of the dissociation dynamics of the Br₂ molecules with CEI, show that information about the evolving molecular structure and electronic geometry can be retrieved from such experiments, therefore paving the way towards the study of complex non-adiabatic dynamics in molecules through XUV time-resolved photoion and photoelectron spectroscopy.